A sonic boom is made by the sudden release of pressure that is built up by the
shock wave. The change in pressure is small—similar to the pressure change you
feel when you descend a few floors in an elevator. But because the change
happens very, very quickly, you hear a loud boom. Not all sonic booms reach the
ground, but those that do arrive less than one minute after flyover and
generally last less than one second.
As shock waves spread across the landscape, sonic booms are continuously
created along the flight path. If you continued to move with the plane at the same
supersonic speed, you'd hear continuous booms. Otherwise, you hear the boom
created when the plane is overhead.